Optical, magnetic and magneto-optic media are primary sources of high performance storage technology that enable high storage capacity coupled with a reasonable price per megabyte of storage. Areal density, typically expressed as billions of bits per square inch of disk surface area (gigabits per square inch (Gbits/in2)), is equivalent to the linear density (bits of information per inch of track) multiplied by the track density in tracks per inch. Improved areal density has been one of the key factors in the price reduction per megabyte, and further increases in areal density continue to be demanded by the industry.
In the area of optical storage, advances focus on access time, system volume, and competitive costing. Increasing areal density is being addressed by focusing on the diffraction limits of optics (using near-field optics), investigating three dimensional storage, investigating potential holographic recording met hods and other techniques.
Polymeric data storage media has been employed in areas such as compact disks (CD) and recordable or re-writable compact discs (e.g., CD-RW), and similar relatively low areal density devices, e.g. less than about 1 Gbits/in2, which are typically read-through devices requiring the employment of a good optical quality substrate having low birefringence.
Referring to FIG. 1, a low areal density system i is illustrated having a read device 3 and a recordable or re-writable storage media 5. The storage media 5 comprises conventional layers, including a data layer 7, dielectric layers 9 and 9′, reflective layer 11, and protective layer 13. During operation of the system 1, a laser 15 produced by the read device 3 is incident upon the optically clear substrate 17. The laser passes through the substrate 17, and through dielectric layer 9, the data layer 7 and a second dielectric layer 9′. The laser 15 then reflects off reflective layer 11, back through the dielectric layer 9′, data layer 7, dielectric layer 9, and substrate 17 and then it is read by the read device 3.
Unlike the CD, storage media having high areal density capabilities, typically up to or greater than about 5 Gbits/in2, employ first surface or near field read/write techniques in order to increase the areal density. For such storage media, although the optical quality of the substrate is not relevant, the physical and mechanical properties of the substrate become increasingly important. For high areal density applications, including first surface applications, the surface quality of the storage media can affect the accuracy of the reading device, the ability to store data, and replication qualities of the substrate. Furthermore, the physical characteristics of the storage media when in use can also affect the ability to store and retrieve data; i.e. the axial displacement of the media, if too great, can inhibit accurate retrieval of data and/or damage the read/write device.
The above issues associated with employing first surface, including near field, techniques have been addressed by utilizing metal, e.g., aluminum, and glass substrates. A disadvantage of using such substrates is the difficulty of creating a pattern, e.g., pit or groove structure, on the surface to facilitate tracking of the read/write device or definition of a sector structure. Once a pattern has been created on the surface, the desired layers are disposed upon the substrate using various techniques, such as, e.g., sputtering. Possible layers include reflective layers, dielectric layers, data storage layers and protective layers. In the case of magnetic or magneto-optic data storage devices, a patterned structure may also be added in the data layer through magnetic read/write techniques, resulting in decreased area available for data storage.
As is evident from the fast pace of the industry, the demand for greater storage capacities at lower prices, the desire to have re-writable discs, and the numerous techniques being investigated, further advances in the technology are constantly desired and sought. What is needed in the art are advances in storage media that enable storage media to be utilized in first surface, including near field, applications.